The present invention relates to a ceramic diaphragm structure and a method for manufacturing the ceramic diaphragm structure. Particularly, the present invention relates to a ceramic diaphragm structure having a specific configuration of a thin diaphragm portion and a method for manufacturing such a ceramic diaphragm advantageously.
There has conventionally been used a diaphragm structure as a member or the like constituting various kinds of sensors. Attention has recently been paid to the diaphragm structure as a member constituting a piezoelectric/electrostrictive actuator. The diaphragm structure includes a substrate having at least one window portion and a thin diaphragm plate formed of flexible, filmy material covering the window portion. When the diaphragm structure is used as a member constituting a sensor, the diaphragm structure is constituted so that a bending displacement caused by an object to be measured by a diaphragm portion is detected by an appropriate means. When the diaphragm structure is used as a member constituting a piezoelectric/electrostrictive actuator, the diaphragm structure is used under the condition that the diaphragm portion is transformed by a piezoelectric/electrostrictive element, causing a pressure in a hollow portion formed inside the diaphragm structure.
Such a diaphragm structure is produced by unitarily combining a substrate with a filmy member which gives a diaphragm. In view of reliability, thermal resistance, anti-corrosive ability and the like, it was considered to constitute such a diaphragm structure by unitarily firing a ceramic. The present inventors have disclosed in Japanese Patent Laid-Open 63-292032 and Japanese Patent Laid-Open 5-49270 a pressure detecting apparatus or a piezoelectric/electrostrictive actuator, in which a ceramic diaphragm structure obtained by unitary firing was used.
Such a ceramic diaphragm structure is generally obtained by firing a unitary laminate composed of (1) a ceramic green substrate having a predetermined configuration and a window portion and (2) a thin ceramic green sheet covering the window portion. However, the present inventors found that there are some problems during a unitary firing operation that a diaphragm portion, which is formed of a ceramic green sheet and positioned on the window portion of the ceramic green substrate, deforms into a depressed shape (The shape that a diaphragm portion protrudes in the opposite direction of the window portion is a protruded shape.) or has cracks. Such a depression or a crack in the diaphragm portion hinders a function or an operation of the diaphragm and makes the reliability of the diaphragm deteriorate.
Additionally, in the case of such a ceramic diaphragm structure, it is usually tried to make a diaphragm portion plane. However, in such a plane diaphragm portion, there are some problems. It is difficult to increase the resonance frequency and to thin the portion because the strength is not satisfactory. Further, an electrode film or a piezoelectric/electrostrictive film formed on the surface cannot be satisfactorily sintered.
Therefore, the present inventors have previously disclosed a thin ceramic diaphragm structure having a high reliability in Japanese Patent Application 6-122733. The diaphragm structure is free from a depression, a crack, or the like in a diaphragm portion, and a resonance frequency of the diaphragm can be increased. Further, the diaphragm structure is excellent in strength, and various films formed on the surface of the diaphragm portion can be satisfactorily sintered without being hindered.
The ceramic diaphragm structure was produced by controlling a difference of sintering speed and a firing shrinkage rate between a ceramic green substrate and a thin ceramic green sheet covering a window portion. However, when a substrate is composed of many structures each having a plurality of window portions, a diaphragm portion sometimes has a depression or has a variance of the height of protrusion.
The present invention has been achieved under such circumstances as a background. An object of the invention is to provide a thin ceramic diaphragm structure having a high reliability. Even if the diaphragm structure has a substrate composed of many diaphragm structures each having a plurality of window portions (diaphragm portions), the diaphragm portions do not have any depression or crack, and a resonance frequency can be increased. The diaphragm portions are excellent in strength, and sintering of various kinds of films formed on the surface of the diaphragm portions. Another object of the invention is to provide a method for producing such a diaphragm structure advantageously.
One aspect of the invention is to provide a ceramic diaphragm structure comprising: a ceramic substrate having at least one window portion; and a thin ceramic diaphragm plate laminated so as to cover the window portion, wherein the ceramic diaphragm structure is unitarily formed so that the thin ceramic diaphragm portion is protruded in the direction opposite to the window portion, and a ceramic connecting layer connects the thin ceramic diaphragm plate to the ceramic substrate.
The ceramic diaphragm plate is preferably composed of a material having, as a main component, a stabilized zirconia, a partially stabilized zirconia, alumina, or a mixture thereof.
An average crystal grain size of each of the ceramic substrate, the ceramic diaphragm plate, and the ceramic connecting layer is desirably 5 xcexcm or less. Further the diaphragm portion has a thickness of 30 xcexcm or less. The ceramic connecting layer has a thickness of 50 xcexcm or less. The diaphragm portion is composed of a dense material having a relative density of at least 90%.
Another aspect of the present invention is to provide a method for producing a ceramic diaphragm structure, comprising the steps of: (a) preparing a ceramic green substrate; (b) forming a ceramic green connecting layer having a predetermined thickness on the ceramic green substrate; (c) forming at least one window portion on the ceramic green substrate on which the ceramic green connecting layer is formed; (d) preparing a thin ceramic green sheet having a predetermined thickness; (e) preparing a unitarily formed laminate by laminating the ceramic green sheet on the ceramic green substrate having at least one window portion so as to cover the window portion in the side of ceramic green connecting layer; and (f) firing the laminate to obtain a unitarily sintered body in which a thin diaphragm portion is formed in a window portion of the ceramic green substrate, and simultaneously with the firing of the laminate, the diaphragm portion is protruded in the direction opposite to the window portion.
In such a method, the ceramic green substrate, the ceramic green sheet, and the ceramic green connecting layer are adjusted so as to have a mid-sintering temperature and a shrinkage rate satisfying the formulae:
S(substrate)xe2x88x92S(sheet)xe2x89xa7xe2x88x920.08{T(substrate)xe2x88x92T(sheet)}xe2x88x921 
0xe2x89xa6T(substrate)xe2x88x92T(sheet)xe2x89xa6300 
S(substrate)xe2x88x92S(sheet)xe2x89xa620 
and
300xe2x89xa7T(substrate)xe2x88x92T(connecting layer)xe2x89xa720 
or
xe2x88x92350xe2x89xa6T(substrate)xe2x88x92T(connecting layer)xe2x89xa6xe2x88x9250 
[S(substrate) and S(sheet) denote shrinkage rates (%) in the direction along the surfaces when the ceramic green substrate and the ceramic green sheet are independently fired at the same temperature as a firing temperature of the laminated body. T(substrate), T(sheet), and T(connecting layer) denote firing temperatures (xc2x0 C.) when a shrinkage rate (%) in the direction along the surface reaches 70% when the ceramic green substrate, the ceramic green sheet, and the ceramic green connecting layer are independently fired at the same temperature as a firing temperature of the laminated body.]
The ceramic green sheet is preferably formed of a material which contains a completely stabilized zirconia material, a partially stabilized zirconia material, alumina material or a material containing, as a main component, a mixture thereof, an average particle diameter of each material ranging from 0.05 to 1.0 xcexcm. The material may contain 30% or less of a sintering aid, for example, clay, silica, magnesia, transitional metal oxide, or the like.